Chemical-mechanical polishing slurry with improved defectivity

ABSTRACT

The copper CMP in a damascene structure composes of copper removal step, which removes majority or all of the redundant copper, and subsequent barrier removal step. The embodiment of the present invention includes the removal of the barrier layer with a slurry blended from abrasive silica particles, chemical materials, and de-ionized water. The blended slurry is aged for at least 4 days before use to achieve an improved defect performance.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the field of chemicalmechanical polishing (CMP) process used to fabricate semiconductordevices. Specifically, the invention relates to the treatment of slurryused in CMP processes where copper is used as interconnecting materialin the semiconductor devices.

[0003] 2. Description of Related Art

[0004] Over last decade, copper has gradually become the interconnectingmaterial of choice for the integrated circuits in semiconductor industrydue to its lower resistivity and better resistance to electro-migration.Due to the difficulty of etching copper using plasma, the method ofdamascene is typically used to create the copper interconnect on chip.When the trench and via are formed simultaneously on the wafer, theprocess is called dual damascene, and when the trench or via are formedseparately on the wafer, the process is called single damascene. In atypical damascene structure, trench and/or via is etched into dielectriclayer, barrier (typically Ta, TaN) and seed copper are deposited intothe trench and/or via, and bulk copper is deposited by electroplating.Typically, the electro-chemically plated copper not only filled thedesired area on the wafer (trench and/or via), it also over burden onthe other area, leaving an un-planarized wafer surface. Chemicalmechanical polishing (CMP) is used to remove the redundant copper and toplanarize the wafer surface. Copper CMP is a multi-step process. Duringthe 1^(st) step, a slurry of high selectivity of copper to barrier isused in the removal of a majority or all redundant copper from the wafersurface. The high selectivity of copper removal rate to barrier removalrate is designed so the polish can stop on the barrier layer. Hence, thenon-uniformity form electro-chemical deposition will not be transferredinto the final copper thickness variation. However, after the highselectivity slurry removes the copper layers, the copper layer disposedinside the trenches also may become polished with the high selectivitycopper-polishing slurry. This over-polishing of the copper layerdisposed inside the trenches causes a depression of the copper layer,such that the copper layer becomes uneven, an effect which is known asdishing. In the 2^(nd) step, a barrier slurry is used to polish off thebarrier. In a low selectivity slurry (LSS) integration scheme, thebarrier polish removes barrier material, dielectric material, and copperat similar rates to improve process margin and to reduce dishing.Sometime, a 3^(rd) step, called buff step, is also used to improvedefectivity on the wafer.

[0005] The defectivity in copper CMP is one of the most difficultchallenges due to the softness of the copper film. The defectivity fromcopper CMP is mainly scratch, sometime residue coming from the degradedpolishing pads, mostly when a soft pad is used to buff out scratches.The defectivity in copper CMP is a major concern because it causes yieldlost, and some reliability concern. Effort has been focused to reducethe copper CMP defectivity. Slurry filtration, including filtration in are-circulation loop and point of use, is routinely used. However, thebenefit of slurry filtration is limited. If a very aggressive filter ischosen, the filter removes not only the unwanted largeparticles/agglomerates, it might also remove the primary abrasiveparticles suspended in the slurry, hence change the polishcharacterization. Also, over aggressive filtering of slurry causes thesheering of the slurry, introducing more defects. It also clogs thefilter easier to make it much less manufacturable. Soft pads are alsoused to buff out the micro-scratches at the top surface. However, thesoft pad has lower planarization efficiency, relative shorter life, andgenerates residue during its degradation.

[0006] Therefore, a need has arisen for processes that overcome theseand other shortcomings of the related art.

SUMMARY OF THE INVENTION

[0007] In accordance with an embodiment of the present invention thebarrier is polished with improved defectivity during the CMP processwith an aged barrier slurry.

[0008] In an embodiment of the present invention, the treatment ofslurry used to polish copper barrier includes the steps of mixing thecomponents of the slurry, aging the mixture for at least five days, andpolishing the wafer with the aged slurry.

[0009] In another embodiment of the present invention, the mixed slurryis re-circulated in a slurry supply loop for a period of at least fivedays before used for polishing.

[0010] Other features and advantages will be apparent to persons ofordinary skill in the art in view of the following detailed descriptionof the invention and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a more complete understanding of the present invention, needssatisfied thereby, and the features and advantages thereof, referencenow is made to the following descriptions taken in connection with theaccompanying drawings.

[0012]FIGS. 1a-1 d are cross-sectional schematics depicting generalconcept of the utilization of copper CMP.

[0013]FIG. 2 illustrates an improved method according to one embodimentof the present invention.

[0014]FIGS. 3a-3 b are the data obtained from aging study wherein FIG.3a is a plot of Sum of Defect (SOD) vs. age in days and FIG. 3b is achart of age, sample size, average defect count, and standard deviation.

[0015]FIG. 4 illustrates copper defect improvement due to slurry aging,demonstrated in a manufacturing environment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0016] Referring to FIGS. 1a-1 d, a schematic diagram demonstrating theconcept of copper CMP is provided. For the sake of simplicity, onlysingle damascene with trench is illustrated, although the same conceptapplies to the dual damascene structures. A wafer substrate with priorlayer of interconnect (102) is provided. The dielectric layer (106) isalso provided. The wafer is patterned and etched, trench (103), (104) isformed. Barrier layer (110) and copper seed layer (112) is deposited.The wafer is then subjected to electro-chemical plating, and copper(120) is deposited and fill the trench. The copper profile in FIG. 1crepresents two typically filling characteristics, conformal filling withwide trench (103) and bottom-up filling with narrow trench (104). CMP isthen used to planarize the surface. In the 1^(st) step of CMP, a slurryof high selectivity of copper to barrier is used in the removal of amajority or all redundant copper from the wafer surface. The highselectivity of copper removal rate to barrier removal rate is designedso the polish can stop on the barrier layer. Hence, the non-uniformityform electro-chemical deposition will not be transferred into the finalcopper thickness variation. However, after the high selectivity slurryremoves the copper layers, the copper layer disposed inside the trenchesalso may become polished with the high selectivity copper-polishingslurry. This over-polishing of the copper layer disposed inside thetrenches causes a depression of the copper layer, such that the copperlayer becomes uneven, an effect which is known as dishing. The resultingwafer is illustrated in FIG. 1d. In the 2^(nd) step, a barrier slurry isused to polish off the barrier. In a low selectivity slurry (LSS)integration scheme, the barrier polish removal barrier, dielectricmaterial, and copper at similar rates to improve process margin andreduce dishing. Sometime, a 3^(rd) step, called buff step is also usedto improved defectivity on the wafer.

[0017] In the prior art, it is suggested that a slurry composition thatincludes Electrapolish (Rodel electrapolish, blended from RodelCUS1201A, Rodel CUS1201B, and DI water) is stable for about 1 week aftermixing, but it is preferable to mix fresh every day. It is alsogenerally believed that as a slurry ages, the contaminants (unwantedparticle existing in the slurry) tend to grow and/or coalesce, causingsubstantial defectivity on the polished wafer. Contrary to that,applicants' have discovered that it is better for use after aging theslurry for at least about 5 to 6 days. It is also found that polishingperformance is not sensitive to the degradation of oxidizer due toaging. The impact of oxidizer degradation can be further minimized byadding a stabilizing agent, which reduces the degradation rate of theoxidizer.

[0018] In accordance with one embodiment of the present invention amethod of providing an improved slurry used for copper barrier CMPprocessing as illustrated in FIG. 2 comprises the Step 1 of mixingabrasive part and chemical part of said slurry and Step 2 of aging saidabrasive part and chemical part that has been mixed for at least fivedays or the equivalent thereof before being used for copper barrier CMPprocessing. In one embodiment the abrasive part is colloid silica (SiO2)and the chemical part contains a corrosion inhibitor such as BTA((Benzotriazole). In Step 3 of FIG. 2, the aged slurry is used forcopper barrier polishing.

[0019] In accordance with a preferred embodiment of the presentinvention the slurry is Rodel electrapolish, blended from RodelCUS1201A, Rodel CUS1201B, and DI (de-ionized ) water. In particular theslurry composition includes Electrapolish that is aged for at least sixdays. During this time the slurry may be mechanically stirred andre-circulated in a slurry supply loop. It is further recognized that theaging may be enhanced by an elevated temperature above normal roomtemperature, mechanical stirring and re-circulation.

[0020]FIGS. 3a and 3 b demonstrate the improvement of defectivity bybarrier slurry aging. The electro-chemically plated blank copper waferswere used in the experiment to monitor the defectivity improvement. Theexperiment conditions are listed below: Equipment: Applied MaterialsMirra Polisher Polish pad: Rodel IC1010 on all three platens MonitorWafers: 12,000 A copper Pilots, annealed at 400 C. Defect Metrology:KLA-Tencor SP1 Slurry: Rodel CUS1201 or electrapolish, blended by volumepartA:partB:DIW = 33%:50%:17%. The blended slurry is aged by days asindicated before polishing. Slurry Delivery: Drawn by peristaltic pumpfrom buckets behind tool, without point of use filter.

[0021]FIG. 4 is a demonstration of defectivity improvement by barrierslurry aging. Each data point in the graph represents a run to qualifythe defectivity performance of the polisher. The Vertical axisrepresents the sum of defects (SOD) on polished blank copper pilotsmeasured by SP1 with 0.24 um sensitivity. The horizontal axis is thedate, when the qualification run was performed in the format of YYMMDD.The qualification run is typically performed daily. The polishingcondition used to collect data on FIG. 4 is similar to that outlined forFIGS. 3a and 3 b except the slurry being delivered from loop, with aloop filter (Mykrolis, Planargard, CMP701E06) and POU filter (Mykrolis,Solaris, SLR0313E1). The day tank is re-filled with slurry when it isdropped below a designated level. The refilling slurry is blended beforerefill. The arrow in the graph indicate the onset of the using of theaged slurry. A clear improvement of defectivity can be observed.

[0022] While the invention has been described in connecting withpreferred embodiments, it will be understood by those of ordinary skillin the art that other variations and modifications of the preferredembodiments described above may be made without departing from the scopeof the invention. Other embodiments will be apparent to those ofordinary skill in the art from a consideration of the specification orpractice of the invention disclosed herein.

What I claim is:
 1. A method of providing a slurry used for CMP processcomprising the steps of: mixing abrasive part and chemical part of saidslurry and aging said mixture at least five days or the equivalentbefore polishing a wafer using the aged slurry.
 2. The method of claim1, wherein said CMP process is copper CMP barrier removal.
 3. The methodof claim 1, wherein said abrasive part is silica (SiO2).
 4. The methodof claim 1, wherein said abrasive part is a colloid silica.
 5. Themethod of claim 1, wherein said chemical part contains a corrosioninhibitor.
 6. The method of claim 1, wherein said chemical part containsa corrosion inhibitor know as BTA (Benzotriazole).
 7. The method ofclaim 1, wherein said slurry is Rodel electrapolish, blended from RodelCUS1201A, Rodel CUS1201B, and DI water.
 8. The method of claim 1,wherein said aged slurry composition includes Electrapolish. aged for atleast six days.
 9. The method of claim 1, wherein said equivalent agingprocess is by one or more accelerated steps of elevated temperature,mechanical stirring or re-circulation in a slurry supply loop.
 10. Amethod of CMP processing a wafer using a slurry comprising the steps of:mixing abrasive part and chemical part of said slurry; aging saidmixture at least five days or the equivalent thereof by an acceleratingstep; and polishing the wafer using the aged slurry.
 11. The method ofclaim 10, wherein said CMP process is copper CMP barrier removal. 12.The method of claim 10, wherein said abrasive part is silica (SiO2). 13.The method of claim 10, wherein said abrasive part is a colloid silica.14. The method of claim 10, wherein said chemical part contains acorrosion inhibitor.
 15. The method of claim 10, wherein said chemicalpart contains a corrosion inhibitor know as BTA (Benzotriazole).
 16. Themethod of claim 10, wherein said slurry is Rodel electrapolish, blendedfrom Rodel CUS1201A, Rodel CUS1201B, and DI water.
 17. The method ofclaim 10, wherein said aged slurry composition includes Electrapolish.aged for at least six days.
 18. The method of claim 10, wherein saidequivalent aging process is by one or more accelerated steps of elevatedtemperature, mechanical stirring or re-circulation.